Lithium polymer batteries can be reduced in thickness and have a high degree of freedom in shape selection, and because of their high electrolyte solution retaining ability, the possibility of leakage of the electrolyte solution is low. For these and other such reasons, the lithium polymer batteries attract attention in a wide range of applications from electric power sources for portable devices to large scale applications such as vehicle-driving electric power sources and stationary storage batteries. Accordingly, improvements of the battery properties are demanded.
Under such circumstances, examples of the important technical problems include the improvement of safety, the improvement of high temperature storage characteristics, and the improvement of cycle characteristics. Among these, the cycle characteristics have been improved by devising the polymer materials and the like used for gel electrolyte in various manners. For example, there is proposed an improvement based on the mixing of a physical cross-linking type polymer and a chemical cross-linking type gel electrolyte (see, for example, Patent Literature 1). The cycle characteristics have also been improved by methods of suppressing the decomposition reaction of the gel electrolyte by forming a protective film on an electrode surface. For example, Patent Literature 2 and Patent Literature 3 describe the improvement of the cycle characteristics by forming a stable film on the electrode surface using a cyclic disulfonic acid ester as an additive for the gel electrolyte.
Additionally, there have been made various proposals on electrode materials, shapes, production conditions, and materials such as additives.